An automotive body includes major structural members such as vehicle longitudinal members that are disposed along a vehicle front-back direction and vehicle widthwise members that are disposed along a vehicle widthwise direction. The vehicle longitudinal members and the vehicle widthwise members are joined, in the way that one type of members have flanges formed at the ends and are joined to the other type of members via the flanges, to ensure rigidity required for the automotive body and bear the load in case of a collision event.
The structural members such as the vehicle lengthwise members and the vehicle widthwise members are required to have properties such as high load transfer capability in the axial direction, high flexural rigidity, and high torsional rigidity. High load transfer capability in the axial direction means that deformation is small when the load acts in the axial direction. High flexural rigidity means that deformation is small against the bending moment when the longitudinal axis of the member is bent, and high torsional rigidity means that the angle of torsion is small against the torsional moment when the member is twisted around the longitudinal axis of the member. In recent years, a high tension steel sheet having a tensile strength of 390 MPa or more (a high-strength steel sheet or a high-tensile steel sheet) has tended to be used as a material for the structural member in an aim to reduce vehicle weight and improve collision safety.
For example, a floor cross member, which is used to reinforce the floor of an automotive body, has a cross section substantially shaped like a gutter and is joined to vehicle longitudinal members such as side sills via outward flanges formed at both ends of the floor cross member. It is important for the floor cross member to have an increased joining strength to other members and an increased rigidity to ensure the rigidity of an automotive body and better load transfer capability when an impact load is applied. Accordingly, it is necessary not only to increase the material strength but to modify the shape of the member so as to improve the load transfer capability and the rigidity when an impact load is applied.
Patent Literature 1 discloses a structural member for an automotive body manufactured by press forming. The structural member has a substantially gutter-shaped cross section as a whole and a groove-like depression in the hat top that is a part corresponding to the bottom in the gutter-shaped cross section.